advanced current mirrors and opamps · wide-swing current mirrors 2 it is proven that if both of...

Advanced Current Mirrors and Opamps

Hossein Shamsi

1

Wide-Swing Current Mirrors

2

It is proven that if both of the following conditions are satisfied, then all transistors will be biased in the saturation region.

efftn

effout

nVVVnV 1

Typically n is chosen identical to 1. So the output swing will be:

effout VV 2

Enhanced Output-Impedance Current Mirrors

3

ArrgR dsdsmout 1211

Implementation of Enhanced Output-Impedance Current Mirror

4

13 effefftnout VVVV 2

33211

dsmdsdsmout

rgrrgR

Advantage:•High Output-ImpedanceDisadvantages:•Low Output-Swing•Imprecise Current Mirror


5

13 effefftnout VVVV

23

3211ds

mdsdsmout

rgrrgR

Advantages:•High Output-Impedance•Precise Current MirrorDisadvantage:•Low Output-Swing


6

12 effout VV 2

33211

dsmdsdsmout

rgrrgR Advantages:•High Output-Impedance•High Output-SwingDisadvantage:•Imprecise Current Mirror

Wide-Swing Current Mirror with Enhanced Output-Impedance

7

12 effout VV 2

33211

dsmdsdsmout

rgrrgR

Advantages:•High Output-Impedance•Precise Current Mirror•High Output-SwingDisadvantage:•High Power Consumption

Modified Wide-Swing Current Mirror with Enhanced Output-Impedance

8

12 effout VV 2

33211

dsmdsdsmout

rgrrgR

Advantages:•High Output-Impedance•Precise Current Mirror•High Output-Swing

Current Mirror Symbol

9

Folded-Cascode Opamp

10

•This opamp is useful when we want to drive capacitive loads.•One of the most important parameters of this modern opamp is its transconductance value.•Therefore, some designers refer to this modern opamp as Operational Transconductance Amplifier (OTA).

Remember the wide-swing current mirror

11

Opamp Gain

12

imsc

sc

im vgiii

vgi2

2

22

13661088 dsdsdsmdsdsmout rrrgrrgr

outmVscouto rgAirv 1

Competition between Two Current Sources

13

If Ibias1>Ibias2 Q1:Triode Q3:Active I=Ibias2

If Ibias2>Ibias1 Q1:Active Q3:Triode I=Ibias1

Assume that:

43

21

LW

LW

LW

LW

Slew Rate

14

Assuming Ibias2>ID4 and Vin+>>Vin-L

D

CISR 4

Assuming Ibias2>ID4 and Vin->>Vin-L

D

CISR 4

Lemma

15

9

1

mgR

Wide-swing current mirror

Frequency Response

16

38

29

15

2

1

10

1

1111

1

pm

pm

pm

p

L

mta

outm

Loutp

Cg

Cg

Cg

Cg

rgACr

Another Schematic for Folded-Cascode Opamp

17

tpBouttnB VVVVV 12

tnefftpBcmieffefftn VVVVVVVV 61121

Output swing:

Input common-mode range:

Folded-Cascode Opamp without wide-swing current source

18

Folded-Cascode Opamp(pmos-input)

19

Folded-Cascode Opamp(pmos-input)

20

What is the DC voltage of the output node?

21

In a single-ended opamp, the DC voltage of the output node is determined by the feedback circuit around the opamp.

dcVout

inbiasbiasout VVRRVV

1

2

Fully Differential Folded-Cascode Opamp

22

The bias voltages Vb1, Vb4, and Vb5 must be chosen so that the following relation is satisfied!!!ID9=0.5ID12+ID3

The opamp is completely symmetric. So we can use the half-circuit of the opamp for AC analysis.

Fully Differential Folded-Cascode Opamp(half-circuit)

23

pm

p

L

imta

outim

Loutp

Cg

Cg

rgACr

7

2

0

1

11

1

What is the DC voltage of the output nodes?

24

In a single-ended opamp, the DC voltage of the output node is determined by the feedback circuit around the opamp.

dcVout

CMxoooxCMoCM

oCM

xCM

VVVVVVVVVRIVV

RVVI

222

The DC voltage of the output nodes can not be determined!!!

CMii VVV

Common-Mode Feedback Circuit(CMFB Circuit)

25

In a fully differential opamp, the CMFB circuit is employed inside the opamp to adjust the common-mode voltage of the output nodes identical to a predetermined voltage, Vref.

CMxo VVVKCLandKVL 2

refoo VVVCircuitCMFB

2

2CMref

x

refoo

VVV

VVV

Ideal CMFB Circuit (1)

26

Vref denotes the desired output common-mode voltage.


27



28


Single-ended Telescopic Opamp

imsc vgi 1

866244 dsdsmdsdsmout rrgrrgr

outmVscouto rgAirv 1

Slew Rate

30L

SS

CISRSR

Frequency Response

31

36

27

13

2

0

1

1111

1

pm

pm

pm

p

L

mta

outm

Loutp

Cg

Cg

Cg

Cg

rgACr i

i

Input common-mode range and output swing

32

tpBouttnB VVVVV 21

3191 effBcmieffefftn VVVVVV

Output swing:

Input common-mode range:

Telescopic Opamp without wide-swing current source

33

Fully Differential Telescopic Opamp

34

The bias voltages Vb3 and Vb4 must be chosen so that the following relation is satisfied!!!ID7=0.5ID9The opamp is completely symmetric. So we can use the half-circuit of the opamp for AC analysis.

Fully Differential Telescopic Opamp(half-circuit)

35

pm

p

L

mta

outm

Loutp

Cg

Cg

rgACr

3

2

1

10

1

11

1


36



37


Fully Differential Two-Stage Opamp(nmos input)

38

L

mp

M

mta

mm

CCg

Cg

RgRgA

1

52

1

25110

C1 denotes the parasitic capacitance.

Fully Differential Two-Stage Opamp(pmos input)

39

40

Fully Differential Two-Stage Opamp(First Stage: Telescopic Cascode)

C1 denotes the parasitic capacitance at node X.Miller compensation method is utilized.

L

mp

M

mta

mm

CCg

Cg

RgRgA

1

92

1

29110

41

Gain-Boosting Opamp

It is proven that the auxiliary amplifier will not affect the performance of the main amplifier if the following relation is satisfied.

AmplifierMainAmplifierAuxiliary UGBWUGBW

42

Gain-Boosting Opamp

43

Differential Pair as an Auxiliary Opamp

44

Folded-Cascode Circuit as an Auxiliary Opamp

45

Comparison of Performance of Various Opamp Topologies

advanced current mirrors and opamps · wide-swing current mirrors 2 it is proven that if both of...

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